1. Field of the Invention
The invention relates generally to the non-invasive diagnosis of conditions within a human or animal body and, more particularly, the invention relates to diagnostic techniques that use the acoustic characteristics within a body to detect respiratory conditions therein.
2. Description of Related Technology
One particularly problematic respiratory condition is pneumothorax. Generally speaking, pneumothorax refers to the formation of a gas cavity between one or both lungs and the chest wall. As is well known, pneumothorax has many potential causes, including, for example, spontaneous rupture of small alveoli or blebs, progression of inflammatory diseases, complications of diagnostic or therapeutic procedures, penetrating wounds caused by a knife, bullet, etc. and blunt chest trauma, which may be, for example, caused by motor vehicle accidents. Although trauma is a significant cause of pneumothorax, severe chest wall injury is often difficult to detect based on the outward appearance of a patient""s body and, as a result, the diagnosis of pneumothorax is often missed in these cases.
Pneumothorax also occurs in 5-15% of mechanically ventilated patients, and other iatrogenic pneumothoraces are becoming more common with the increasing use of chest invasive procedures such as central venous line insertions, which are often used for monitoring and fluid replacement in emergency trauma cases, and percutaneous transthoracic lung biopsies. For these invasive procedures, the pneumothorax rates are about 5% and 20%, respectively. It is estimated that over 50,000 cases of pneumothorax occur each year in the United States and, thus, more effective diagnosis of pneumothorax could significantly reduce morbidity and mortality.
Conventional pneumothorax diagnostic techniques are typically based on patient history, physical examination of the patient, chest x-rays (CXRs), computerized tomogram (CT) and ultrasound. Patient history, physical examination and CXRs are the techniques most commonly employed to diagnose pneumothorax. Unfortunately, patient history and physical examination are typically unreliable techniques for diagnosing pneumothorax because the symptoms associated with pneumothorax are also present in a number of unrelated clinical conditions such as cardiac ischemia, pneumonia, pulmonary embolism, esophageal spasm/reflux, and musculoskeletal strain. As a result, diagnosis of pneumothorax based on patient history and/or physical examination is very difficult and, in many cases, virtually impossible. For example, one study reported that physical examinations resulted in misdiagnosis in 42% of patients having a pneumothorax condition that arose from a penetrating chest wound.
Percussion is one common physical examination technique used by physicians to diagnose a variety of chest abnormalities. Most studies of percussion rely on qualitative descriptions such as xe2x80x9cdullxe2x80x9d and xe2x80x9cresonantxe2x80x9d to describe the chest sounds resulting from a percussive input to the patient""s chest. Reported percussion response waveforms of a normal chest are typically 20 milliseconds (ms) long and contain an initial spike followed by a decaying waveform with spectral peaks in the 70 Hertz (Hz) xe2x88x92200 Hz range. Using percussion, skilled physicians have noted xe2x80x9chyperresonancexe2x80x9d as an acoustic phenomenon that is often heard in patients having a pneumothorax condition. In addition, acoustic asymmetries with large pneumothoraces have been reported when manually percussing both clavicles in turn while auscultating (i.e., listening to) the sternum. In any event, despite widespread belief in the usefulness of percussive techniques, uncertainty of its diagnostic capability exits because of the inherent dependence on the skill of the operator and their personal perception of the sound qualities of a patient""s chest response.
Misdiagnosis of pneumothorax may also occur when using CXRs and CT due to large bullae and cysts within the lung or pleural space, patient clothing, tubing, skin folds, and chest wall artifacts. Additionally, with CXRs, patients are exposed to potentially harmful doses of radiation. Unfortunately, the radiation problem is compounded by the fact that CXRs are often performed unnecessarily (which needlessly exposes patients to radiation) because physicians are unwilling to miss the diagnosis due to the life threatening nature of pneumothorax, its tendency to progress rapidly to tension pneumothorax and the ease with which pneumothorax can be treated if detected. As a result, CXRs are ordered as a precautionary measure for many patients that do not actually have pneumothorax. Further, because each patient with pneumothorax is typically subjected to multiple CXRs to generate subsequent films that document relative improvement, it is estimated that the total number of pneumothorax diagnostic tests conducted each year in the U.S. may be hundreds of thousands.
To overcome the diagnostic limitations of CXRs and CT, patients may be placed in the upright or lateral decubitus positions, and/or end-expiratory exposures may be used instead. Unfortunately, these positioning maneuvers are typically difficult to perform on critically ill patients. In addition to patient positioning difficulties, a common limitation of CXRs and CT is the difficulty and danger of transporting a critically ill patient to the imaging suite and the lack of equipment and staff availability in a timely manner, which is typically the case at night or in remote areas (such as, for example, battlefield conditions, the scene of an accident, a bedside, etc.). Further, CXRs, CT and other conventional imaging techniques typically involve a significant amount of delay between the examination of a patient and the availability of diagnostic results. Such a delay may be unacceptable in many situations, particularly where the patient""s condition is critical or life-threatening. Still further, as is commonly known, diagnostic techniques based on ultrasound suffer from a high false positive rate due to inherent limitations.
Some researchers have used zero radiation techniques that rely on external low frequency forcing to non-invasively diagnose lung diseases other than pneumothorax. For example, Wodicka et al. [Wodicka GR, Aguirre A, DeFrain PD, and Shannon DC, Phase Delay of Pulmonary Acoustic Transmission from Trachea to Chest Wall, IEEE Transactions on Biomedical Engineering 1992; 39:1053-1059] and Kraman et al. [Kraman SS, Bohandana AB, Transmission to the Chest of Sound Introduced at the Mouth, J Applied Physiology, 1989;66:278-281] studied acoustic transmission characteristics from the trachea to the chest wall by introducing low frequency sound waves at the mouth and measuring the sound waves received at the chest Wall. The Wodicka et al. study found that geometrical changes within the lung cause sound transmission times to be frequency dependent because different wavelengths of sound couple to different parts of the lung lining. The Kraman et al. study found that changes in the lung volume or the resident gas composition did not consistently alter the peak-to-peak amplitude or the peak frequency of the measured signal. On the other hand, Donnerberg et al. [Donnerberg RL, Druzgalski CK, Hamlin RL, Davis GL, Campbell RM, Rice DA. British J, Diseases of the Chest 1980;74:23-31] studied the sound transfer function in normal and congested dog lungs using a technique similar to that described by Wodicka et al. and found a consistent increase in the transmitted sound as the lung wet-to-dry weight ratio increased.
Another abnormal respiratory condition that typically occurs in patients in ambulances and operating rooms is the misplacement of an endotracheal (ET) tube within a patient""s trachea. As is generally known, ET tubes are placed in patients to establish an open airway, deliver anesthetic agents, and/or to perform mechanical ventilation. Typically, when an ET tube is misplaced, it travels too far into one of the two main bronchi (i.e., left and right) and blocks the other bronchus partially or completely, thereby limiting or eliminating ventilation into the lung associated with the obstructed bronchus. ET tube misplacement may also occur after the ET tube has been initially properly placed. For example, the ET tube may spontaneously move due to movements of the patient and/or movements of the ventilator tubing attached to the ET tube. Additionally, an ET tube may be misplaced into the esophagus of a patient or may be misplaced as a result of extubation.
Typically, ET tube placement is checked using x-ray or carbon dioxide measurements. However, carbon dioxide based detection techniques provide limited accuracy and the time, cost and radiation exposure associated with x-rays limits the usefulness of x-ray based detection of ET tube misplacement, especially when multiple or on-line monitoring of the ET tube placement is desired.
Diagnostic techniques are provided to enable the detection of a respiratory condition within a patient""s body. Generally speaking, the diagnostic techniques described herein use the acoustic characteristics of a patient""s lungs and chest to determine if a respiratory condition is present. More specifically, the diagnostic techniques described herein compare the acoustic generation and transmission characteristics of the patient""s chest and lungs to reference acoustic characteristics and/or predetermined threshold values to determine if an abnormal respiratory condition is present within the patient. In particular, the diagnostic techniques described herein can be used, for example, to detect the presence of a gas cavity between one or more of a patient""s lungs and chest wall, which is symptomatic of a pneumothorax condition. Alternatively, the diagnostic techniques described herein can be used, for example, to detect a relative difference between the acoustic transmission characteristics from a patient""s trachea to the left and right lungs, which is symptomatic of an ET tube blocking (or partially blocking) one of the patient""s bronchi.
In accordance with one aspect of the invention a system and method for detecting a respiratory condition within a body emits sound waves into a first location of the body and converts the emitted sound waves into a first electrical signal. The system and method receives vibrations resulting from the sound waves interacting with the respiratory condition and impinging on a second location of the body, converts the received vibrations into a second electrical signal and uses the first and second electrical signals to calculate a value indicative of the respiratory condition.
Additionally, the system and method may generate a first set of frequency data using the first electrical signal and may further generate a second set of frequency data using the second electrical signal. The system and method may calculate transfer function data using the first and second sets of frequency data and may use the transfer function data to calculate an energy ratio indicative of the respiratory condition.
In some embodiments, the system and method may calculate the energy ratio indicative of the respiratory condition based on a first energy within a first band of frequencies and a second energy within a second band of frequencies. Still further, the system and method may define the first band of frequencies to include higher frequency components than the second band of frequencies.
In accordance with another aspect of the invention, a system and method for detecting a respiratory condition within a body receives indigenous respiratory sounds adjacent to a first location of the body at a first time and converts the indigenous respiratory sounds received at the first time into a first electrical signal. Additionally, the system and method generates a first set of frequency data using the first electrical signal and uses the first set of frequency data to calculate an energy ratio indicative of the respiratory condition.
Still further, the system and method may calculate the energy ratio indicative of the respiratory condition based on a first energy within a first band of frequencies and a second energy within a second band of frequencies. In some embodiments, the system and method may define the first band of frequencies to include higher frequency components than the second band of frequencies.
In accordance with still another aspect of the invention a system and method of detecting a respiratory condition within a body impacts a portion of the body and receives vibrations resulting from the impact interacting with the respiratory condition and the impacted portion of the body. Further, the system and method converts the received vibrations into an electrical signal and uses the electrical signal to calculate a value indicative of the respiratory condition.
In some embodiments, the system and method calculates an envelope of the electrical signal and uses the envelope to calculate a characteristic of the envelope of the electrical signal. The system and method may calculate the characteristic of the envelope of the electrical signal by identifying a temporal location associated with a maximum amplitude of the envelope of the electrical signal, identifying a portion of the envelope of the electrical signal surrounding the temporal location associated with the maximum amplitude of the envelope of the electrical signal and calculating the characteristic of the envelope of the electrical signal using the identified portion of the envelope of the electrical signal.